Wafer holder protecting wafer against electrostatic breakdown

ABSTRACT

A wafer holder includes a mounting plate for mounting a wafer and having a plurality of fluid holes, a decompressor for absorbing the wafer toward the mounting plate while sucking air between the mounting plate and the wafer through the fluid holes, and a liquid supply unit for supplying a liquid toward the wafer to release the wafer from the wafer holder. The liquid has a specific resistance lower than the specific resistance of water.

BACKGROUND OF THE INVENTION

[0001] (a) Field of the Invention

[0002] The present invention relates to a wafer holder mounting thereon a wafer while protecting the wafer against electrostatic breakdown, and more particularly, to a wafer holder such as used for transferring a wafer from a product line to a treatment station such as a polishing device.

[0003] (b) Description of the Related Art

[0004] A wafer includes a plurality of semiconductor devices fabricated thereon. If the wafer has electric charge on the main surface on which the semiconductor devices having fine elements are mounted, the electrostatic force of the electric charge may damage the interconnects formed on the wafer or the resist film coated on the wafer.

[0005] Patent Publication JP-A-6-123924 and JP-A-11-289004, for example, describe wafer holders used for transferring wafers from product lines to polishing devices.

[0006] In the wafer holder described in JP-A-6-123924, a detachment device or release member supplies a positive pressure gas between the wafer holder and the wafer mounted thereon by absorption, to thereby release the wafer, wherein the positive pressure gas is ionized air. The ionized air neutralizes the electrostatic charge attached onto the wafer.

[0007] In the wafer holder described in JP-A-11-289004, a release member supplies water or a mixed gas including water between the holder and the wafer mounted thereon to release the wafer therefrom. The thus supplied water has a polarity that cancels the residual electric charge on the interface between the holder and the wafer.

[0008] The present inventors studied the conventional wafer holders as described above, and have found there is some possibility that electrostatic breakdown of a wafer occurs in the conventional wafer holders due to the electrostatic charge formed on the wafer by friction etc. More specifically, each of the ionized air used in JP-A-6-123924 and the water used in JP-A-11-289004 has a large specific resistance which may be unable to drain the electrostatic charge and protect the wafer against the electrostatic breakdown thereof.

SUMMARY OF THE INVENTION

[0009] In view of the above, it is an object of the present invention to provide a wafer holder which mounts thereon a wafer and is capable of effectively protecting the wafer against the electrostatic breakdown especially during releasing the wafer therefrom.

[0010] The present invention provides a wafer holder including: a mounting plate for mounting thereon a wafer; at least one fluid hole formed on the mounting plate; a fluid exhaustion unit selectively communicating with the fluid hole for sucking fluid through the fluid hole for exhaustion; and a liquid supply unit selectively communicating with the fluid hole for supplying a liquid through the fluid hole, the liquid having a specific resistance lower than a specific resistance of water.

[0011] The present embodiment also provides a method for treating a wafer by using a mounting plate having a fluid hole, the method including the steps of: absorbing the wafer to the mounting plate while sucking air between the wafer and the mounting plate through the fluid hole; and releasing the wafer from the mounting plate while supplying a liquid having a specific resistance lower than a specific resistance of water.

[0012] In accordance with the wafer holder and the method of the present invention, the liquid having a specific resistance lower than the specific resistance of water effectively protects fine elements on the wafer against the electrostatic breakdown of the fine elements, due to the lower specific resistance which effectively discharge the electrostatic charge from the wafer during releasing the wafer from the mounting plate of the wafer holder by using the liquid.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a schematic block diagram of a wafer holder according to an embodiment of the present invention.

[0014]FIG. 2 is a perspective view of the mounting plate shown in FIG. 1, depicting also a wafer by a dotted line.

[0015]FIG. 3A is a perspective view of another example of the mounting plate, and FIG. 3B is an enlarged perspective view of the nozzle shown in FIG. 3A.

PREFERRED EMBODIMENTS OF THE INVENTION

[0016] In one embodiment of the present invention, the wafer holder is used for a chemical mechanical polishing (CMP) device. The CMP device receives a wafer from the product line through the wafer holder, places the wafer in a polishing head while centering the wafer therewith, and then polishes the wafer.

[0017] The wafer holder is used to absorb the wafer for mounting the same thereon, and to release the wafer from the absorbed state by using the liquid.

[0018] In one embodiment of the present invention, when a handling robot places the wafer onto the mounting plate of the wafer holder, the fluid exhaustion member communicates selectively with the fluid hole and operates for absorption of the wafer through the fluid hole by sucking the air around the wafer holder through the fluid hole and exhausting the air from the fluid exhaustion member.

[0019] The wafer holder then releases the wafer from the absorption for centering the wafer with the CMP device. In this stage, the liquid supply unit selectively communicates with the fluid hole and supplies the liquid onto the wafer on the mounting plate through the fluid hole. Thus, the wafer is released from the mounting plate and floats on the liquid staying on the mounting plate.

[0020] The liquid supply unit prepares in advance the liquid having a lower specific resistance than water in a mixing device. The liquid is supplied to the fluid hole by using a regulator, which supplies at a specified pressure and a specified flow rate of the liquid. The fluid hole supplies the liquid onto the surface of the wafer opposing the wafer holder.

[0021] By supplying the liquid having a lower specific resistance than water through the fluid hole, the fine elements formed on the wafer are protected by the liquid against the electrostatic breakdown. It is preferable that the liquid have a specific resistance equal to or lower than 0.5MΩ-cm, which allows the fine elements on the wafer to be scarcely electrified by electrostatic charge irrespective of the pressure or the flow rate of the liquid supplied through the fluid hole.

[0022] Aqueous carbon dioxide or aqueous ammonium may be preferably used as the liquid having a lower specific resistance than water. Carbon dioxide does not affect the health of the human body, and is easy for handling without need of a special treatment for disposal thereof. The aqueous ammonium is also easy for handling without need of a special treatment for disposal thereof. Carbon dioxide is more preferable because it does not diffuse an odor.

[0023] In a preferred structure of the present invention, a plurality of fluid holes are provided on the mounting surface of the mounting plate. A lower pressure and a lower flow velocity of the liquid are preferable for protecting the fine elements on the wafer in view of lower possibility of the electrostatic breakdown thereof. On the other hand, a lower flow rate of the liquid may cause a slow down of the movement of the wafer when the wafer floats off the mounting plate upon supply of the liquid from the fluid hole. In this case, it may occur that a certain time length is needed for the wafer to float off the mounting plate after the fluid hole starts for supply of the liquid, or that the transfer of the wafer is failed due to incomplete floating of the wafer off the mounting plate.

[0024] By storing and slowly supplying the liquid having a specific resistance lower than water by using a regulator and the plurality of fluid holes, electrification of the wafer is suppressed. The plurality of fluid holes reduce the flow velocity of the liquid upon supplying the liquid at the specified flow rate which is sufficient for floating the wafer off the mounting plate at a desired speed.

[0025] It is preferable that the plurality of fluid holes be arranged on the mounting plate so that each fluid hole has an elongate opening extending in a radially outward direction. In an alternative, the fluid holes may be arranged at a constant density on the mounting plate. These configurations allow the pressure of the liquid to be maintained at a lower value, and enhance the speed of the wafer floating off the mounting plate.

[0026] Each fluid hole may have a shape of funnel in the vicinity of the surface of the mounting plate, thereby reducing the flow rate of the liquid per unit area of the fluid hole to further suppress the electrification of the wafer.

[0027] A plurality of nozzles may be provided in the wafer holder of the present invention, each of the nozzles having a plurality of fluid hole. This configuration reduces the flow rate of the liquid per unit area of the wafer, suppresses the electrification of the wafer and allows the wafer to float off the mounting plate at a higher speed.

[0028] The wafer holder may have a pure water supply unit and another regulator disposed downstream of the pure water supply unit for controlling the pressure and the flow rate of the pure water. The wafer holder selects either the liquid from the liquid supply unit or the pure water from the pure water supply unit.

[0029] When a liquid is supplied through the fluid holes, the polishing head of the CMP device may be washed by the wafer holder. Although the pressure and the flow rate of the liquid should be controlled during release of the wafer from the mounting plate in view of protection of the fine elements on the wafer, the pressure and the flow late need not be controlled during washing the polishing device. Rather, a higher pressure is preferable in the washing of the polishing head at a higher speed.

[0030] Now, the present invention is more specifically described with reference to accompanying drawings. Referring to FIG. 1, a wafer holder, generally designated by numeral 10, according to an embodiment of the present invention is used for a polishing device by transferring the wafer between the polishing device and a product line for manufacturing wafers.

[0031] A wafer 12 is transferred from the product line to the holder 10 by a handling robot, positioned at the holder 10 by a positioning tool 14, absorbed and hanged by a polishing head 16 which is pivotable around a vertical axis and movable in the vertical direction, and then transferred to the CMP device. The holder 10 absorbs and holds the wafer 12 positioned thereat, then releases the same from the absorption, thereby transferring the wafer 12 between the product line and the CMP device.

[0032] In the present embodiment, the main surface of the wafer 12 on which fine elements are formed opposes to the holder 10, and the bottom surface of the wafer 12 opposes the polishing head 16. However, the main surface and the bottom surface of the wafer 12 may oppose the polishing head 16 and the holder 10, respectively.

[0033] The holder 10 includes a mounting plate 18 having a planar top surface. The mounting plate 18 has a plurality of fluid holes on the top surface thereof, the fluid holes communicating with a tube 22 extending downwardly from the bottom surface of the mounting plate 18. The fluid holes are selectively communicated with a fluid exhaustion unit 28, a liquid supply unit 30 or a pure water supply unit 22 via the tube 22, a first selector valve 24 and a second selector valve 26.

[0034] If both the first selector valve 24 and the second selector valve 26 select respective first positions, then the fluid holes communicate with the fluid exhaustion unit 28. If the first selector valve 24 and the second selector valve 26 select the first position and a second position, respectively, then the fluid holes communicate with the pure water supply unit 32. If the first selector valve 24 selects the second position, then the fluid holes communicate with the liquid supply unit 30.

[0035] The fluid exhaustion unit 28 has a function for absorbing the air around the top of the mounting plate 18 through the fluid holes. The fluid exhaustion unit 28 communicates with an air decompressor or vacuum pump which reduces the internal pressure of the fluid exhaustion unit 28. The liquid and solid particles absorbed in the fluid exhaustion unit 28 is collected by a drain trap of the air decompressor and discharged outside the system. In the present embodiment, air, nitrogen or inert gas is selectively introduced into the tube 22 via a third selector valve 34.

[0036] The liquid supply unit 30 includes a mixing device 36 and a regulator 38. The mixing device 36 prepares aqueous carbon dioxide having a specific concentration by dissolving carbon dioxide into pure water. The carbon dioxide may be replaced by another gas.

[0037] The aqueous carbon dioxide used in the present embodiment has a specific resistance of 0.5MΩ-cm or lower. Aqueous carbon dioxide having a specific resistance of 0.5MΩ-cm or lower effectively prevents electrification of the fine elements on the wafer 12 irrespective of the pressure and the flow rate of the liquid supplied from the fluid holes.

[0038] The regulator 38 is disposed between the mixing device 36 and the first selector valve 24. The regulator 38 has functions for storing the aqueous carbon dioxide prepared by the mixing device 36 and for supplying the stored aqueous carbon dioxide at a specified pressure and a specified flow rate through the tube 22 to the fluid holes.

[0039] The pure water supply unit 32 has a function for supplying pure water from a pure water reservoir (not shown) to the fluid holes via the tube 22 and the first selector valve 24. The pure water supply unit 32 supplies through the tube 22 the pure water for polishing use in the polishing head 16. It is to be noted that another regulator is provided within the system of the pure water supply unit 32, and that the pressure and the flow rate of the pure water is controlled by the another regulator.

[0040] Referring to FIG. 2, the mounting plate 18 has the plurality of fluid holes 20 on the top surface of the mounting plate 18. The cross section of each fluid hole 20 forms an elongate slit extending in a radially outward direction. The plurality of elongate fluid holes are arranged on the mounting plate 18 in four directions having an angle of 90 degrees with respect to one another.

[0041] For protecting fine elements on the wafer 12 against the electrostatic breakdown during supplying the liquid through the fluid holes 20 from the liquid supply unit 30, the pressure and the flow velocity of the liquid should be reduced. In the present embodiment, by providing the plurality of fluid holes 20 on the mounting plate 18, the liquid is uniformly supplied from the fluid holes 20 toward the wafer 12 to thereby reduce the pressure and the flow velocity of the liquid. The plurality of fluid holes 20 assure a sufficient flow rate to be obtained for floating the wafer 12 off the mounting plate 18 at a desired speed.

[0042] For reducing the pressure of the liquid in the vicinity of the top opening of the fluid hole 20, it is preferable that the fluid hole 20 have a larger cross section toward the top opening of the fluid hole 20 along the flow direction of the liquid.

[0043] Referring to FIG. 3A, another example of the mounting plate 19 for use in the wafer holder of FIG. 1 has a plurality of nozzles 21 each having a top flush with the top surface of the mounting plate 19. Referring to FIG. 3B, each nozzle 21 has a plurality of fluid holes 20′ on top of the nozzle 21. Each fluid hole 20′ of the nozzle 21 communicates with the tube 22 in FIG. 1 at the bottom of the nozzle 21. When the liquid supply unit 30 supplies liquid through the tube 22 to the nozzles 21, the liquid is supplied toward the main surface of the wafer 12 through the respective fluid holes 20′ of each nozzle 21 at a lower flow velocity.

[0044] The fluid holes 20′ formed in each of the plurality of nozzles 21 reduce the flow rate per unit area of the main surface of the wafer 12, whereby the flow velocity toward the wafer 12 is lowered to suppress electrification of the wafer 12, without delaying the floating speed of the wafer 12 off the mounting plate 18. Thus, the wafer 12 is quickly transferred to the subsequent production stage.

[0045] Hold and release of the wafer 12 by the wafer holder 10 in association with the CMP device is detailed hereinafter with reference to FIG. 1. After a preceding wafer is transferred from the CMP device to the product line, the positioning tool 14 is in a release state for the wafer. The mounting plate 18 is positioned apart from the polishing head 16 in the vertical direction. The first selector valve 24 and the second selector valve 26 are at their neutral positions, wherein the tube 22 is isolated from either of the fluid exhaustion unit 28, liquid supply unit 30 and pure water supply unit 32.

[0046] After the second selector valve 26 selects the second position, with the first selector valve 24 selecting the first position, the pure water supply unit 32 communicates with the fluid holes 20 through the tube 22. In this state, pure water is supplied from the fluid holes 20 toward the polishing head 16 at a relatively high pressure to clean the polishing head 16 quickly.

[0047] After the second selector valve 26 switches to the first position, the pressure for the mounting plate 18 and the pure water supply path is reduced, and the remaining pure water in the system is discharged from the mounting plate 18 and the pure water supply path. The plurality of fluid holes 20 uniformly formed on the mounting plate 18 allow the pure water to be discharged with ease from the top surface of the mounting plate 18. The remaining pure water is stored in the drain trap and eventually discharged therefrom. After a specified time length elapses, the second selector valve 26 returns to the neutral position.

[0048] Subsequently, the handling robot places a next wafer 12 onto the mounting plate 18. The second selector valve 26 then selects the first position to communicate the fluid holes 20 with the fluid exhaustion unit 28. The mounting plate 18 absorbs the main surface of the wafer 12 for holding the wafer 12 on the mounting plate 18.

[0049] The first selector valve 24 then selects the second position to communicate the tube 22 with the regulator 38, whereby the liquid supply unit 28 supplies aqueous carbon dioxide in the regulator 38 at the specified pressure and the specified flow rate to the fluid holes 20. The fluid holes 20 supply the aqueous carbon dioxide between the mounting plate 18 and the wafer 12 to float the wafer 12 off the mounting plate 18.

[0050] The positioning tool 14 determines the position of the wafer 12 then floating off the mounting plate 18 with respect to the polishing head 16. Subsequently, the first selector valve 24 selects the first position to communicate the fluid holes 20 with the fluid exhaustion unit 28, thereby exhausting the aqueous carbon dioxide between the mounting plate 18 and the wafer 12. The wafer 12 is thus absorbed by the mounting plate 18 at the main surface of the wafer 12, followed by release of the wafer 12 by the positioning tool 14.

[0051] Subsequently, the mounting plate 18 is raised upward. The polishing head 16 has at the bottom thereof a membrane or diaphragm which expands toward the mounting plate 18. Upon raising the mounting plate 18 upward, the wafer 12 is contacted with the expanded membrane of the polishing head 16. Subsequently, the polishing head 16 hangs the wafer 12 by absorption. When the polishing head hangs the wafer 12, the first selector valve 24 selects the second position substantially at the same time, whereby aqueous carbon dioxide is supplied through the fluid holes 20 to release the wafer 12 from the absorption by the mounting plate 18. Thus, the wafer 12 is transferred from the mounting plate 18 to the polishing head 16.

[0052] Subsequently, the polishing head 16 pivots for polishing the wafer 12. After the wafer 12 is polished by the polishing head 16, the polishing head 16 returns the wafer 12 onto the mounting plate 18. In this stage, when the membrane of the polishing head 16 expands, the mounting plate 18 is raised at the same time.

[0053] The second selector valve 26 then selects the first position to communicate the fluid holes 20 with the fluid exhaustion unit 28. The mounting plate 18 absorbs the wafer 12 and then falls downward. The polishing head 16 stops expansion of the membrane and starts for shrinkage of the membrane.

[0054] The first selector valve 24 selects the second position whereby the aqueous carbon dioxide stored in the regulator 38 is supplied to the fluid holes 20 at the specified pressure and the specified flow rate. The aqueous carbon dioxide is supplied between the mounting plate 18 and the wafer 12 from the fluid holes 20, thereby floating the wafer 12 off the mounting plate 18.

[0055] Subsequently, the positioning tool 14 starts for positioning of the wafer 12 on the float off the mounting plate 18 with respect to the polishing head 16. The first selector valve 24 selects the first position to communicate the fluid holes 20 with the fluid exhaustion unit 28, thereby exhausting the aqueous carbon dioxide between the mounting plate 18 and the wafer 12. The wafer 12 is then absorbed by the mounting plate 18 at the main surface of the wafer 12, and the positioning tool 14 releases the wafer 12.

[0056] Finally, the handling robot transfers the wafer 12 from the mounting plate 18 to the product line. In this step, the first selector valve 24 and the second selector valve 26 select the first position and the second position, respectively, to communicate the fluid holes 20 with the pure water supply unit 32. The wafer 12 after being subjected to the polishing process is attached with the polishing liquid. Thus, the wafer 12 floated off the mounting plate 18 by the pure water does not suffer from an electrostatic breakdown. The pure water, however, may be replaced by the aqueous carbon dioxide in this step of floating the wafer 12.

[0057] Since the above embodiments are described only for examples, the present invention is not limited to the above embodiments and various modifications or alterations can be easily made therefrom by those skilled in the art without departing from the scope of the present invention. 

What is claimed is:
 1. A wafer holder comprising: a mounting plate for mounting thereon a wafer; at least one fluid hole formed on said mounting plate; a fluid exhaustion unit selectively communicating with said fluid hole for sucking fluid through said fluid hole for exhaustion; and a liquid supply unit selectively communicating with said fluid hole for supplying a liquid through said fluid hole, said liquid having a specific resistance lower than a specific resistance of water.
 2. The wafer holder as defined in claim 1, wherein said liquid supply unit includes a regulator controlling a pressure and a flow rate of said liquid before supplying said liquid through said fluid hole.
 3. The wafer holder as defined in claim 1, wherein said liquid has a specific resistance of 0.5MΩ-cm or lower.
 4. The wafer holder as defined in claim 1, wherein said liquid is obtained by dissolving carbon dioxide into water.
 5. The wafer holder as defined in claim 1, wherein said liquid includes ammonium and water.
 6. The wafer holder as defined in claim 1, wherein said at least one fluid hole include a plurality of fluid holes.
 7. The wafer holder as defined in claim 1, wherein said fluid hole has a larger cross section as viewed toward an exit of said fluid hole on said mounting plate.
 8. The wafer holder as defined in claim 1, wherein said mounting plate includes a plurality nozzles, each of said nozzles including said plurality of fluid holes.
 9. The wafer holder as defined in claim 1, further comprising a pure water supply unit for preparing pure water, wherein said liquid and said pure water is selectively supplied through said fluid hole.
 10. A method for treating a wafer by using a mounting plate having a fluid hole, said method comprising the steps of: absorbing the wafer to the mounting plate while sucking air between the wafer and the mounting plate through the fluid hole; and releasing the wafer from the mounting plate while supplying a liquid having a specific resistance lower than a specific resistance of water.
 11. The method as defined in claim 10, wherein said liquid has a specific resistance of 0.5MΩ-cm or lower.
 12. The method as defined in claim 10, further comprising the step of supplying pure water through the fluid hole.
 13. A method for chemically-mechanically polishing a wafer by using a polishing head and an associated mounting plate having a fluid hole, said method comprising the steps of: absorbing the wafer to the mounting plate while sucking air between the wafer and the mounting plate through the fluid hole; releasing the wafer from the wafer holder while supplying a liquid having a specific resistance lower than a specific resistance of water; and supplying pure water through the fluid hole to wash the polishing head. 